I have the same problem, ive been trying to figure it out for the last 2 hours and apparently its because i need a to update...something, i really have no idea what. I have an AMD FX 8120 processor and i think thats what needs to be updated, but i cant for the life of me find the update :/

*(This fix is for those people whose game won't launch after the steam is launching program window)*Turns out there were two programs interfering with Portal 2 when I tried to launch. For me it was Norton antivirus (which I thought was deleted off my pc) and the AMD Utility manager. After you do the steps below make sure to run Portal 2. If it launches then a program is interfering with it. I'm going to post a related article and the list of programs known to interfere with Steam as well.